Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials.
Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner.
Certain factors greatly affect the efficiency of CdTe PV modules in terms of cost and power generation capacity. For example, CdTe is relatively expensive and, thus, efficient utilization (i.e., minimal waste) of the material is a primary cost factor. In addition, the energy conversion efficiency of the module is a factor of certain characteristics of the deposited CdTe film layer. Non-uniformity or defects in the film layer can significantly decrease the output of the module, thereby adding to the cost per unit of power. Also, the ability to process relatively large substrates on an economically sensible commercial scale is a crucial consideration.
CSS (Closed System Sublimation) is a known commercial vapor deposition process for production of CdTe modules. Reference is made, for example, to U.S. Pat. No. 6,444,043 and U.S. Pat. No. 6,423,565. Within the vapor deposition chamber in a CSS system, the substrate is brought to an opposed position at a relatively small distance (i.e., about 2-3 mm) opposite to a CdTe source. The CdTe material sublimes and deposits onto the surface of the substrate. In the CSS system of U.S. Pat. No. 6,444,043 cited above, the CdTe material is in granular form and is held in a heated receptacle within the vapor deposition chamber. The sublimated material moves through holes in a cover placed over the receptacle and deposits onto the stationary glass surface, which is held at the smallest possible distance (1-2 mm) above the cover frame.
Since the best film quality of a thin film is achieved in a narrow temperature range just below the point at which the film would begin sublimating off faster than it is depositing (e.g., between about 600° C. to about 650° C. for cadmium telluride), it is desired to keep the substrate temperature between this narrow temperature range throughout the CSS process. However, in a CSS process, the cover must be heated to a temperature considerably greater (e.g., about 800° C. when depositing cadmium telluride) than the substrate to ensure that no material deposits and builds up on the cover. Since the cover is hotter than the substrate, the cover will raise the temperature of the substrate through radiation (e.g., heat exchange) from the cover. This temperature gain can result in a gradient of film quality through the thickness of the film, due to a temperature increase of the substrate during the deposition of the thin film. Further, if the temperature gain of the substrate is too high, the film thickness is limited because the substrate may have become too hot to receive any additional material. This would require that the process begin with the substrate at a lower temperature, resulting in the first film deposited being lower in crystalline quality.
Accordingly, there exists an ongoing need in the industry for an improved vapor deposition apparatus and process for economically feasible large scale production of efficient PV modules, particularly CdTe modules. In particular, a need exists for an improved sublimation plate for use in an economically feasible large scale production of efficient PV modules, particularly CdTe modules, in a CSS process.